The notched tip (called the "arrow nock") of an arrow engages the bow string at the "nocking point". A "string nock" is generally mounted on the string above the arrow nock to locate the nocking point. There are various string nocks in use today.
A single nock serves well if the bow is being drawn with the fingers. But if a bow string release device is used, the geometry of the bow string at full draw relative to the release and arrow puts a substantial force on the release pushing the release against the arrow. The result is that the arrow will fall off the string under either of two conditions: (1) The hunter draws back when he sights an animal and then finds he can't take the shot, becomes tired and lets the bow return to the undrawn position . . . the arrow will almost certainly fall off . . . (2) The archer draws the bow to full draw and, while aiming, lets the arrow creep forward slightly, as most archers do, and just prior to releasing the arrow, he pulls back to full draw again; at this time the arrow will drop off most of the time.
If a clamp-on type of nock is used below the arrow to keep the release away from the arrow, the bow can be drawn without the arrow falling off. But, the metallic, clamp-on nock tends to "chew up" the release and a groove will be worn in the top of the barrel (if the release barrel is made of soft metal or plastic). This will allow the release to slide over the nock, creating unwanted pressure on the arrow nock and which leads to arrow torque and poor arrow flight. If that does not occur, the constant pressure on the string nock from the release will cause the string nock to loosen and slide up to the arrow nock, creating the aforementioned problems. Many times this will cause the string serving to be severed and unraveled. The arrow nock can also be gouged by the string nock.